I've recently updated a nx6325 to SLIC 2.1 with the method described in the PCBeta Forum.

The trick is to start HPQFlash tool and just before clicking on "Next" to start the update the BIOS image has to be changed in the memory of HPQFlash. You only need to exchange the RSA part and the SLIC marker (part with version info). To do that I used HxDen and the SLIC binary available from a thread in this forum. The rest has to be done by yourself.